There are many known power assist steering systems for automotive vehicles. Some provide assist by using hydraulic power and others by using electric power.
Electric power assist steering systems that utilize a rack and pinion gear set provide power assist by using an electric motor to either (i) apply rotary force to a steering input shaft connected to a pinion gear, or (ii) apply linear force to a steering member having the rack teeth thereon. The electric motor in such systems is typically controlled in response to (i) a driver's input torque applied to the vehicle steering wheel and/or (ii) vehicle speed.
In U.S. Pat. No. 3,983,953, an electric motor is coupled to the input steering shaft and energized in response to the torque applied the steering wheel by the vehicle operator. The electronic control system includes a torque sensor and a vehicle speed sensor. A computer receives the output signals provided by both sensors. The computer controls the amount of the assistance provided by the motor dependent upon the applied torque and the vehicle speed.
U.S. Pat. No. 4,415,054, utilizes an electric assist motor having a rotatable armature encircling a steering member having rack teeth thereon. Rotation of the electric assist motor armature causes linear movement of the steering member through a ball nut drive arrangement. A torque sensing device is coupled to the steering column to sense driver-applied input torque to the steering wheel. The torque sensing device uses a magnet/Hall-effect sensor arrangement for sensing rotational deflection across a torsion bar. An electronic control unit monitors the signal from the torque sensing device and controls the electric assist motor in response thereto.
Previous electric assist steering systems have not included elements for detecting electrical failures of the control electronics or the device used to measure driver-applied input torque. Such failures, however, are hazardous since they may cause an undesired electrical drive potential to be appl:ed to the electric assist motor. If this were to occur, the vehicle might turn sharply when the operator did not desire nor expect it to.
Failure of a torque sensor utilizing a Hall-effect device could cause such problems, for example. The Hall-effect device might operate between 0 VDC and 12 VDC, with an output of 6 VDC representing no input torque. When the driver applies input torque to steer the vehicle, the torsion bar twists causing the Hall-effect device to generate a D.C. voltage either greater or less than 6 VDC, depending on the steering direction. If an electrical open or short were to occur in the torque sensing device, the output might jump to 12 volts or drop to zero volts, thus causing the vehicle to hard steer in an undesired and unexpected manner.